钙对氨煤混燃高温贫氧区NH协同煤焦还原NO影响机理

Effect mechanism of calcium on NO reduction in high temperature oxygen-lean zone of ammonia-coal co-firing

氨煤混燃可有效降低火力发电中CO_(2)排放,但氨作为N源与煤混烧时会增加NO_(x)排放,探究氨煤混燃过程中NO还原机理对实现氨煤混燃低氮排放非常必要。采用密度泛函理论探究氨煤混燃高温贫氧区内NO还原机理,进一步分析煤中重要矿物质钙对NH_(3)协同煤焦还原NO的影响。理论计算结果表明,高温贫氧区内氨基/煤焦可将NO通过形成NNH、N_(2)O等重要过渡中间产物进而还原成N_(2),NH协同煤焦还原NO中NNH的形成需克服438.49 kJ/mol能垒,成为体系反应决速步。矿物质钙不利于NH和NO在煤焦表面吸附,降低二者在煤焦表面的吸附能约187.09 kJ/mol。煤焦表面顶部Ca存在下NH还原NO可通过生成中间产物NNH(路径1)和N_(2)O(路径2)2种路径实现,路径1决速步能垒为636.41 kJ/mol,较NH/煤焦/NO体系的决速步能垒高出197.92 kJ/mol;Ca参与下路径2中决速步N_(2)O自由基的形成需455.74 kJ/mol,较NH/煤焦/NO体系的决速步能垒高出17.25 kJ/mol,2条路径均表明金属矿物质钙抑制了NH/煤焦对NO的还原。Ca的存在增强了NNH基团与煤焦表面结合能,使得顶部钙催化作用下路径1较路径2抑制作用更强。采用过渡态理论计算了Ca参与前后NH/煤焦/NO体系决速步动力学参数,结果表明,顶部钙参与下NH协同煤焦还原NO的速率低于无钙参与体系NO的还原速率,且路径1中NH协同煤焦还原NO速率比路径2更低,确定了顶部钙催化作用下路径1较路径2抑制作用更强,动力学结果与热力学结果一致。

The co-combustion of ammonia and coal can effectively reduce CO_(2) emissions in thermal power generation,but ammonia will increase NO_(x) emissions when it is co-fired with coal as an N source.Exploring the NO reduction mechanism in the co-combustion process of ammonia and coal is very necessary in achieving a low nitrogen emission.In this paper,the density functional theory was used to explore the mechanism of NO reduction in the high temperature oxygen depleted zone of ammonia and coal co-combustion,and the effect of calcium in coal,an important mineral,on the reduction of NO by NH_(3) coke in coal was further analyzed.The theoretical calculation results show that the amino/coal coke in the high temperature oxygen depleted zone can reduce NO to N_(2) by forming important transition intermediates such as NNH and N_2O.The formation of NNH in the reduction of NO by NH coke needs to overcome the energy barrier of 438.49kJ/mol,which becomes the decisive step of the system reaction.The mineral calcium was not conducive to the adsorption of NH and NO on the surface of coal coke,and the adsorption energy of the two on the surface of coal coke was reduced by about 187.09 kJ/mol.In the presence of Ca at the top of coal coke surface,the NH reduction NO can be achieved by generating two paths:intermediate products NNH(path 1) and N_2O(path 2),and the deciding step energy barrier of path 1was 636.41 kJ/mol,which was 197.92 kJ/mol higher than that of the NH/coal coke/NO system.With the participation of Ca,the formation of N_(2O) radicals in path 2 required 455.74 kJ/mol,which was 17.25 kJ/mol higher than that of NH/coal coke/NO system,and both paths showed that the metal mineral calcium inhibited the reduction of NH/coal coke to NO.The presence of Ca enhanced the binding energy between the NNH group and the surface of coal coke,making the inhibition of path 1 stronger than path 2 under the catalysis of calcium at the top.The kinetic parameters of the NH/coal coke/NO system before and after Ca participation were calculated by transition state theory.And the results showed that the rate of NH synergistic coal coke reduction NO under the participation of top calcium was lower than that of NO in the calcium-free participation system.The reduction rate of NH synergistic coal coke in path 1 was lower than that in path 2,and it was determined that path 1 had stronger inhibition than path 2 under the top calcium catalysis,and the kinetic results were consistent with the thermodynamic results.